This invention relates to pressure-sensitive adhesive-coated sheet material and is especially concerned with pressure-sensitive tapes, including adhesive transfer tape, which have excellent tack, excellent shear properties at both room temperature and elevated temperatures, and good adhesion to steel and a variety of polymeric substrates.
Three properties which are useful in characterizing the nature of pressure-sensitive adhesives are tack, peel, and shear. The first measures the adhesive's ability to adhere quickly, the second its ability to resist removal by peeling, and the third its ability to hold in position when shearing forces are exerted. Generally speaking, the first two are directly related to each other but are inversely related to the third; cf., e.g., Sanderson U.S. Pat. No. 4,077,926, Col. 1, lines 31-36.
Modern industry presents a number of situations where pressure-sensitive adhesive products having balanced properties (i.e., excellent tack, peel, and shear) would be highly desirable. For example, to attach gasketing and weatherstripping materials in automotive and other industrial applications, there is a need for a pressure-sensitive adhesive which possesses a high tack as well as a high shear under such environmental conditions as will be encountered in use as the result of high and low temperatures, humidities, etc. A typical application involves the application of neoprene door and trunk lid gaskets to automobiles; the gasketing is continually under stress, often at high temperatures, making it essential that the mounting adhesive possess both high shear and high adhesion. Another application involves the lamination of sound-absorbing foam in truck cabs and electronic equipment cabinets; the foam, which is curved to fit the available space, seeks to return to its original dimensions, thereby exerting a constant stress on the adhesive. Still other demanding applications for pressure-sensitive adhesives include the industrial processing of film, fabric, carpet, or paper webs which must be spliced so as to retain their integrity when thereafter passed through high temperature ovens, the lamination of non-slip pads to polyethylene surf boards, the bonding of polyvinyl fluoride wall coverings to bulkheads in passenger aircraft, the adhesion of polyolefinbacked carpet to metallic substrates, etc. As a result of these needs, many persons have tried to prepare a pressure-sensitive adhesive having balanced properties (i.e., excellent tack, peel and shear), especially at elevated temperatures.
It has long been recognized that adhesives that consist essentially of a copolymer of alkyl acrylate and a minor proportion of copolymerizable monomer such as acrylic acid do not require a tackifying resin and are able to resist aging; thus, such adhesives have advantages over the earlier and more traditional rubber-resin adhesives; cf. Ulrich U.S. Pat. No. Re. 24,906. Although acrylate pressure-sensitive adhesives can be prepared as either solutions or aqueous emulsions, the latter are typically employed because of the higher molecular weight and percentage solids attainable at a practical coating viscosity.
The early so-called acrylate adhesives were somewhat heat-sensitive (cf., e.g., Bemmels U.S. Pat. No. 3,617,362 and 3,707,518), and crosslinking has been proposed. Unfortunately, the improved heat resistance and holding power obtained by crosslinking have been generally offset by the loss in tack. Sanderson U.S. Pat. Nos. 3,740,366 and 4,077,926 discuss improving heat resistance of acrylate adhesives by crosslinking with, e.g., zinc octoate or trivalent chromium ion; the adhesion and shear values obtained are not high enough for use in, e.g., the automotive applications discussed above. These patents also suggest that such conventional additives as tackifiers may be included but do not enlarge on this point.
Maska U.S. Pat. No. 3,701,758 discloses a pressure-sensitive adhesive formed from cyanoacrylate polymers which is said to have a balance of tack, adhesion, and cohesion, even at temperatures of 50.degree.-100.degree. C. This adhesive may (but usually does not) include a tackifier, and no mention is made of crosslinking; its low tack and adhesion values render it unsatisfactory for bonding firmly to such low energy surfaces as neoprene or polyethylene. Jubilee U.S. Pat. No. 3,222,419 discloses a terpolymer of vinyl acetate, acrylate or methacrylate, and a crosslinkable resin, the resultant product being said to have a balance of tack, adhesion and cohesion, as well as the ability to adhere to a wide variety of substrates; unfortunately, however, the adhesive has low heat resistance. Hydrogenated rosin ester may be included as a tackifying agent; this adhesive does not, however, perform well at elevated temperatures.
Samour U.S. Pat. Nos. 3,299,010 and 3,400,103 disclose pressure-sensitive adhesives of the acrylate:acrylic acid family which may be cured or crosslinked, suggesting that any of a wide variety of tackifying resins might be included but failing to provide any details; it appears that these patents illustrate the general principle that it has heretofore been possible to improve one adhesive property only at the sacrifice of others. Published European Patent Application EPA 79900506.1 (based on PCT application U.S. 79/00288, filed Apr. 30, 1979, published Nov. 29, 1979) describes an acrylate:acrylic acid adhesive which is prepared in solution and crosslinked with 0.002-0.2 weight percent Cr.sup.+++ to obtain a product having good initial adhesion and internal strength at high temperatures. This adhesive, however, does not have sufficiently high adhesion for use on some or all of such low energy surfaces as acrylonitrile:butadiene:styrene (ABS) terpolymer, low density polyethylene and isotactic polypropylene.
In recent years acrylate-based adhesives that possess a balance of high tack, high peel, and high shear properties have been prepared by coating a blend of such monomers as iso-octyl acrylate and acrylic acid on a backing, maintaining an inert atmosphere, and polymerizing the blend in situ. The commercial preparation of such tape products, however, requires a substantial investment in unconventional manufacturing equipment. Further, even these products have not possessed the desired uniformly excellent adhesion to low energy polymeric substrates.
Prior to the present invention, then, there has been a need for pressure-sensitive adhesive tape products which not only have balanced properties at wide temperature ranges and adhere well to low energy surfaces but also can be prepared easily and economically on conventional equipment.